Several patents and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents and publications is incorporated by reference herein.
Safety laminates have been in commercial production for almost a century and have been utilized in applications that require sheet material having a high degree of clarity and impact resistance. For example, safety laminates have been widely used in the automobile industry as windshields or side windows because the laminate structures are characterized by high impact and penetration resistance and because they do not scatter glass shards and debris when shattered. More recently, safety laminates have also been incorporated into building structures as window, walls, stairs, and the like.
Simple safety laminates typically consist of a sandwich of two glass sheets or panels bonded together with an interlayer of a polymeric sheet. One or both of the glass sheets may be replaced with optically clear rigid polymeric sheets, such as sheets made of polycarbonate. Safety glass laminates have further evolved to include multiple layers of glass and polymeric sheets bonded together with interlayers of polymeric sheets.
The interlayers used in safety laminates are typically made from relatively thick polymer sheets, which exhibit toughness and bondability to the glass in the event of a crack or crash. Widely used interlayer materials include complex, multicomponent compositions based on poly(vinyl butyral), poly(urethane), and ethylene vinyl acetate copolymers.
Ionomers are copolymers produced by partially or fully neutralizing the carboxylic acid groups of precursor or parent polymers that are acid copolymers comprising copolymerized residues of α-olefins and α,β-ethylenically unsaturated carboxylic acids. The use of acid copolymer resins and ionomers in interlayer sheets in safety laminates is known. See, for example, U.S. Pat. Nos. 3,344,014; 3,762,988; 4,663,228; 4,668,574; 4,799,346; 5,759,698; 5,763,062; 5,895,721; 6,150,028; 6,265,054; 6,432,522; and 8,399,097; U.S. Patent Appln. Publn. Nos. 20020155302; 20020155302; 20060182983; 20070092706; 20070122633; 20070289693; 20080044666, and PCT Patent Appln. Publn. Nos. WO9958334; WO2006057771; and WO2007149082.
In this connection, ionomers have been useful in safety laminates intended for structures requiring a high degree of penetration resistance. Some examples include hurricane-resistant glazing and structural elements such as glass staircases and glass balustrades. In particularly demanding application, the use of ionomeric interlayer sheets in safety laminates having ballistic resistance is described in, e.g., U.S. Pat. Nos. 5,002,820 and 7,641,965; and PCT Patent Appln. Publn. No. WO03068501.
There is a continuing need to improve the physical and mechanical properties of interlayers for safety laminates. In particular, there is a need for interlayers that have the exceptional clarity associated with ethylene acid copolymers and their ionomers combined with the improved mechanical properties provided by cross-linking. Cross-linking occurs when chemical bonds are formed between polymeric moieties, producing polymeric networks that can enhance the overall strength of the crosslinked material. Crosslinked polymeric networks often exhibit improved elongation, mechanical integrity, tensile strength and resistance to break, compared with the polymers in an un-crosslinked state.
Various methods for crosslinking polymers are known. For example, ethylene vinyl acetate (EVA) copolymers used to form interlayer sheets and encapsulants for photovoltaic modules are often crosslinked with peroxides. Crosslinking of EVA with peroxide can form gel, however, and can lead to the degradation of the EVA. See, for example, U.S. Pat. No. 6,093,757, issued Jul. 25, 2000, to Pern.
Accordingly, there is a need to develop new methods of crosslinking ethylene acid copolymer resins and their ionomers, in order to improve their mechanical properties while retaining good processability in the melt, good optical properties, and good long-term stability so that they may be used in more advanced safety laminates.